Deformation history of ultra high-pressure ophiolitic serpentinites in the Zermatt-Saas Zone, Créton, Upper Valtournanche (Aosta Valley, Western Alps)

Abstract

Detailed multiscale structural analyses and mapping (1:20 scale) integrated with petrological investigation were used to study a portion of the Zermatt-Saas serpentinites that crop out in upper Valtournanche (north-western Italy). Results are synthesized in a foliation trajectory map that displays the transposed original lithostratigraphy of a serpentinite body exposed at Créton. The serpentinite body comprises magnetite sheets and rare, decimetre-thick, diopsidite layers and lenses. Moreover, veins and aggregates of Ti-chondrodite and Ti-clinohumite, olivine-rich layers and lenses, veinlets of olivine, and layers of dark pyroxenite are embedded in the serpentinites. Serpentinites and associated rocks record three relative age groups of ductile structures: D1 consists of rare folds and S1 foliation; D2 is a group of isoclinal folds and a very pervasive foliation (S2), which is the dominant structure; D3 includes a crenulation and shear zones affecting S2. The detailed meso-structural and microstructural analyses allowed individuating the metamorphic environment of successive deformation stages and correlating the resulting tectono-metamorphic investigation with those already inferred in surrounding areas. In addition, metre- to submillimetre-sized pre-D2 structural, mineralogical, and textural relics have been clearly identified in spite of the strong transposition imposed during the development of S2 high pressure - ultra-high pressure foliation.